How's that work exactly? Say I want a storyline where Jean runs into Victor Creed...do I just use him, ask permission first, ask the Logan player if there is one?
Generally, if a character is closely associated with a PC -- particularly villains -- at a minimum it's best to notify that PC, if not ask for permission. Just because they may have something in the pipeline. But if there's no PC, you can just go ahead and use the character; just try to tread lightly and leave the character with as few strings attached as possible. That way, if a player comes around wanting to play Logan, they don't have to deal with the fact that Creed's now dead or a Green Lantern or something.